U.S. Republican presidential candidate Donald Trump made another gaffe showing his apparent unfamiliarity with certain Christian practices when he and his wife attended church services at the non-denominational First Christian Church in Council Bluffs, Iowa, on Sunday.

When the Communion plates were passed, Trump mistook the silver plates for the offertory, digging out several bills from his pocket, according to the Washington Post, which was based on an Associated Press report. He, his wife and two staffers reportedly took Communion.

When the offering plates were passed minutes later, he again took out some bills and placed them this time on the right plates.

"I thought it was for offering," he later told staffers with a laugh, referring to the Communion plates.

Two weeks ago, Trump also found himself the butt of jokes when he spoke to students at Liberty University, a Christian school in Virginia.

After promising to protect Christians worldwide, Trump read a verse from Second Corinthians — one of the books in the New Testament. But Trump pronounced it "Two Corinthians," as it's often written in text. Critics pounced on this American faux pas, saying it showed that Trump is just pretending to be a knowledgeable Christian.

However, according to the Washington Post, saying "Two Corinthians" is common in Britain, and that Trump's mother is from Scotland.

Trump has long been reaching out to evangelical voters to solidify his lead in the polls by portraying himself as a sincere and dedicated Christian, even if his efforts appear strained.

Last year, Trump spoke about partaking in Holy Communion. "When I drink my little wine — which is about the only wine I drink — and have my little cracker, I guess that is a form of asking for forgiveness, and I do that as often as possible because I feel cleansed," he said, according to CNN. "I think in terms of 'Let's go on and let's make it right.'"

Trump said he was raised and still identifies himself as a Presbyterian, referring to the mainline Presbyterian Church (USA) denomination. Last December, he said, "I am an evangelical. I'm a Christian. I'm a Presbyterian."

However, church leaders in Manhattan, New York where he resides say he is not an active member in any of their churches.

Continuing his effort to show his Christian faith, Trump released a new video on Saturday showing off the Trump family Bible, saying it is "very special to me."

Trump has described the Bible as the greatest book ever written. However, when he was asked what is his favourite verse, he could not say anything.

While campaigning in Iowa last week, he handed out photos from his childhood confirmation from a Presbyterian church.

However, Trump appeared unaware that conservative evangelicals often keep mainline Protestant churches at arms-length because they are seen as more liberal.

Despite his stumbling attempts to woo more Christian voters, many Americans who are religious say they are still supporting Trump even if they don't see him as very religious at all.